In recent years, various kinds of information communication units, such as cellular phone units and portable information terminals, have been developed or have just started to be commercialized all over the world. Each of these information communication units operates based on its unique system. Thus, the frequency bands, the output powers and the modulation types are different from each other among these systems. Consequently, transmission power amplifiers adapted for the respective systems have been developed and integrated into the terminals.
In Japan, two types of communication methods are utilized for automobile radiotelephone units and cellular phone units, i.e., an analog type FM (frequency modulation) type and a digital type .pi./4 shift DQPSK (differential quadrature phase shift keying) type. Both of the analog and the digital types are assigned to the frequency band of 800 MHz, while only the digital type is assigned to the 1.5 GHz band. Furthermore, a cellular phone called "PHS" (personal handy-phone system) is for a digital type .pi./4 shift DQPSK type and the 1.9 GHz band. The output power of automobile radiotelephone units and cellular phone units is on the order of 1 W, whereas that of cellular phones is on the order of 10 mW. Since the former phone units have a cell radius of several kilometers and a hand-over function, the phone units of the former type can be used for communication even during travel in an automobile or the like. On the other hand, the phone units of the latter type have a cell radius of several hundreds of meters and have been developed by such an approach as to utilize conventional indoor-use cordless phone units for outdoor use. Furthermore, though the 2.4 GHz band, i.e., the ISM (Industrial Scientific Medical) band, has been globally assigned to industrial, scientific and medical applications, it is also under consideration to apply the ISM band to wireless LANs (local area networks) within offices, factories or various kinds of sites in accordance with the spread spectrum (SS) type so as to comply with the standard in which the output power is 10 mW/MHz (frequency band: 26 MHz). As can be seen from these tendencies, information communication units, which can be used anywhere at any time, will surely be popular for use in daily life in the near future.
Conventionally, a transmission power amplifier satisfying the desired frequency band and the desired output power is integrated into the corresponding terminal depending upon which of these types the terminal is based on. Thus, a user has been required either to have terminals of different types corresponding to the domestic service areas and applications, or to buy an expensive, large-sized terminal compatible with several different types for itself.
FIG. 35 is a block diagram of a conventional example. This conventional example is a multi-stage power amplifier for transmitting two different radio frequency (RF) signals having different frequency bands and output powers, in which two banks of power amplifiers corresponding to the respective frequency bands are used.
A first power amplifier PA1 includes: a first input matching circuit PA104; a first GaAs MESFET PA101; a first inter-stage matching circuit PA105; a second GaAs MESFET PA102; a second inter-stage matching circuit PA106; a third GaAs MESFET PA103; and a first output matching circuit PA107.
A second power amplifier PA2 includes: a second input matching circuit PA204; a fourth GaAs MESFET PA201; a third inter-stage matching circuit PA205; a fifth GaAs MESFET PA202; a fourth inter-stage matching circuit PA206; a sixth GaAs MESFET PA203; and a second output matching circuit PA207.
The power amplifier of this conventional example is surely compatible with different output powers, modulation types and frequency bands, but requires a larger number of components for a single configuration. Thus, such a power amplifier is contrary to the recent requirement of downsizing a terminal and adversely increases the costs thereof.
FIG. 36 is a simplified circuit diagram of a radio frequency (RF) integrated circuit described in Japanese Laid-Open Patent Publication No. 8-88524 which was laid open (laid-open publication date: Apr. 2, 1996) posterior to the priority date of the present application. This patent publication relates to an RF integrated circuit including an amplifier operating in both of analog and digital types. As shown in FIG. 36, the drain 3604 of an FET 3601 on the last stage of the amplifier is connected to the input terminal of an analog type output matching circuit PC1 and the input terminal of a digital type output matching circuit PC2 via a switch SW1. The output terminal of the analog type output matching circuit PC1 and the output terminal of the digital type output matching circuit PC2 are connected to an output terminal 3605 via a switch SW2. By switching the output matching circuits PC1 and PC2 and selecting one of them via the switches, the operation corresponding to each type can be performed.
FIG. 37 shows a graph (a) showing the variations in distortion D and power added efficiency .eta. (i.e., the ratio of the difference between the input RF power and the output RF power with respect to the DC power supplied to an amplifier) with respect to an input power Pin and a graph (b) showing the variation in output power Pout with respect to the input power Pin.
FIG. 38 is a graph showing the input power dependences of the output matching circuits PC1 and PC2. The axis of the abscissas indicates the input power Pin, while the axis of the ordinates indicates the output power Pout. Moreover, Pn denotes a nominal output power. In the region where the output power varies linearly with respect to the input power, the distortion and the power added efficiency are low. On the other hand, in the region where the output power varies non-linearly with respect to a higher input power, the distortion and the power added efficiency become high. In view of these characteristics, PC1 for the analog type and PC2 for the digital type of the power amplifier are configured so as to have the RF power input/output characteristics shown in FIG. 38. More specifically, in operation, PC1 for the analog type does not require the linearity of the output power with respect to the input power and is configured so as to have a high power added efficiency (i.e., subjected to an efficiency matching). On the other hand, PC2 for the digital type is configured so as to ensure the linearity of the output power with respect to the input power (i.e., subjected to a distortion matching) such that a distortion is not generated in an RF signal passing through the amplifier during the operation. Then, the power added efficiency thereof becomes lower than that of the analog type.
Examining the disclosure of Japanese Laid-Open Patent Publication No. 8-88524, the integrated circuit described therein can be regarded as being compatible with the analog and the digital signals in the same frequency band (900 MHz) and with the same output power. Thus, in the above-described integrated circuit, the output matching circuits PC1 and PC2 thereof are subjected to an impedance matching such that the loss of a transmitted signal is minimized with respect to the same frequency band. Therefore, in the case of transmitting RF signals in different frequency bands, the impedances cannot be matched with each other, thereby increasing the loss. As a result, the integrated circuit has a problem in that a desired output power and a desired distortion cannot be obtained in such a case. Furthermore, assume that the output powers are different from each other (e.g., a power on the order of 1 W and a power on the order of 100 mW are processed by PC1 and PC2, respectively). Then, in order to output a power on the order of 100 mW by using an active element capable of outputting a power on the order of 1 W, a mechanism for controlling the input power of the FET is indispensable. If the active element is operated on the order of 100 mW by reducing the input power under such a control mechanism, then the power added efficiency is extremely decreased as compared with the operation on the order of 1 W, thereby adversely increasing the power consumption. As a result, when an information communication unit is driven by a battery, the longevity of the battery is disadvantageously shortened. In addition, such a power amplifier is not compatible with RF signals, the frequencies and the output powers of which are both different from each other. Furthermore, in case where such a power amplifier is used as an integrated part of an information communication unit having transmission and reception functions, some selection means for switching a signal to be transmitted by the power amplifier and a signal received via an antenna is required to be provided. Nevertheless, this point is not referred to in the above-cited patent publication. In other words, the correlation between the transmission function of a power amplifier for switching the output matching circuits in accordance with the analog and the digital types and the transmission/reception functions of an information communication unit is not mentioned in the above-cited patent publication.
In view of these problems, the present invention has objectives of providing a power amplifier which is commonly applicable to different systems used for various kinds of information communication units (i.e., the systems having different frequency bands, output powers to be transmitted and modulation types) and can be formed in a reduced size at lower costs, and providing a highly value-added communication unit by using such a power amplifier.